I have a 18,000 vinyl pool with algae. Six days ago (before vacation) I shocked the pool raising the cl to over 20-ppm with a pH around 7.4. This morning, pool has more algae then before I shocked & CL was 5-6-ppm. Is there an algecide product I should try or am I not raising the cl high enough when shocking?

I replaced the sand in my filter a couple weeks ago, after I converted the pool from Baqucil with zeobrite (zeolite).

Total alk = 200
CYA = is a little low

How do I get rid of this algae?